For expression purposes recombinant genes are usually transfected into the target cells, cell populations or tissues, as cDNA constructs in the context of an active expression cassette to allow transcription of the heterologous gene. The DNA construct is recognized by the cellular transcription machinery in a process that involves the activity of many trans-acting transcription factors (TF) at cis-regulatory elements, including enhancers, silencers, insulators and promoters (herein globally referred to as “promoters”).
Gene promoter are involved in all of these levels of regulation, serving as the determinant in gene transcription by integrating the influences of the DNA sequence, transcription factor binding and epigenetic features. They determines the strength of e.g. transgene expression which is encoded by a plasmid vector as well as in which cell type or types said transgene will be expressed.
The most common promoters used for driving heterologous gene expression in mammalian cells are the human and mouse cytomegalovirus (CMV) major immediate early promoter. They confer a strong expression and have proved robust in several cell types. Other viral promoters such as the SV40 immediate early promoter and the Rous Sarcoma Virus (RSV) long-terminal-repeat (LTR) promoter are also used frequently in expression cassettes.
Instead of viral promoters, cellular promoters can also be used. Among known promoters are those from house-keeping genes that encode abundantly transcribed cellular transcripts, such as beta-actin, elongation factor 1-alpha (EF-Ialpha), or ubiquitin. Compared to viral promoters, eukaryotic gene expression is more complex and requires a precise coordination of many different factors.
One of the aspects concerning the use of endogenous regulatory elements for transgene expression is the generation of stable mRNA and that expression can take place in the native environment of the host cell where trans-acting transcription factors are provided accordingly. Since expression of eukaryotic genes is controlled by a complex machinery of cis- and trans-acting regulatory elements, most cellular promoters suffer from a lack of extensive functional characterization. Parts of the eukaryotic promoter are usually located immediately upstream of its transcribed sequence and serves as the point of transcriptional initiation. The core promoter immediately surrounds the transcription start site (TSS) which is sufficient to be recognized by the transcription machinery. The proximal promoter comprises the region upstream of the core promoter and contains the TSS and other sequence features required for transcriptional regulation. Transcription factors act sequence-specific by binding to regulatory motifs in the promoter and enhancer sequence thereby activating chromatin and histone modifying enzymes that alter nucleosome structure and its position which finally allows initiation of transcription. The identification of a functional promoter is mainly dependent on the presence of associated upstream or downstream enhancer elements.
Another crucial aspect concerning the use of endogenous regulatory elements for transgene expression is that some promoters can act in a cell specific manner and will lead to the expression of the transgene on in cells of a specific type or, depending on the promoter, in cells of a particular subset.
Therefore, one goal of the present invention is to obtain new sequences suitable for expressing recombinant genes in mammal cells with high expression levels and in a cell type specific manner.
Such sequence address a need in the art for retinal cells specific promoter to develop systems for the study of neurodegenerative disorders, vision restoration, drug discovery, tumor therapies and diagnosis of disorders.